WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
There are some sweep tubes that have G3 separate from the cathode connection. Is there any advantage to having a small positive voltage on G3 instead of grounding it? On the tubes that have G3 separate, some of the tube sheets suggest 30 or so volts.
I would say that G3 is another electrode to play with. Who knows how many watts it can dissipate before it blows up. Perhaps G3 drive? Perhaps a place to inject feedback?
More grids is equal to more variables = combinations and permutations.
Plan out a few what ifs and hook one up to a curve tracer.
All just for fun!